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Postprint
This is the accepted version of a paper presented at IEEE international conference on
robotics and automation, ICRA 2001, Seoul, South Korea, 21-26 May, 2001.
Citation for the original published paper:
Lilienthal, A J., Wandel, M., Weimar, U., Zell, A. (2001)
Sensing odour sources in indoor environments without a constant airflow by a mobile
robot
In: Proceedings: IEEE international conference on robotics and automation (pp.
4005-4010). IEEE
https://doi.org/10.1109/ROBOT.2001.933243
N.B. When citing this work, cite the original published paper.
Permanent link to this version:
Sensing Odour Sources in Indoor Environments Without a
Constant Airflow by a Mobile Robot
Achim Lilienthal, Andreas Zell
University of T¨ubingen, WSI-RA,
Sand 1,
D-72076 T¨ubingen
f
lilien,zell
g@informatik.uni-tuebingen.de
Michael Wandel, Udo Weimar
University of T¨ubingen, IPC,
Auf der Morgenstelle 8,
D-72076 T¨ubingen
f
mw,upw
g@ipc.uni-tuebingen.de
Abstract
A mobile robot with gas sensors commonly denoted as ’electronic nose’ enables some interesting appli-cations including the development of an ’electronic watchman’ that is able to detect and localize odour sources. This paper describes the assembly of a mobile odour sensing system and presents investi-gations on its practical operation in an indoor en-vironment without a constant airflow1. Lacking a constant airflow leads to a problem which cannot be neglected in real world applications. In this case the response of the used metal oxide gas sensors is dom-inated by air turbulence rather than concentration differences. In the following article we show that this kind of problem could be overcome by driving the robot with a constant speed, thus adding an extra constant airflow relative to the gas sensors location. If the robot’s speed is not too low the described sys-tem proved to be well suited to detect even weak odour sources.
Because driving with constant speed is an indis-pensable condition to perform the basic tasks of a mobile odour sensing system, a new localization strategy is proposed, which takes this into account.
1
Introduction
The ability of an autonomous mobile robot to inter-act with a dynamic environment is based upon its ability to sense relevant qualities of the surround-ing world. Different types of sensors are available to perform this task. Their design is often inspired by biological principles: For instance, sensors based on three of the five human senses are widely used
1This work is part of the project ’Senses for Mobile Robots’
which is supported within a pinpoint program by the state of Baden-W ¨urttemberg.
enabling robots to ’see’ (camera systems), ’hear’ (microphones) and ’feel’ (tactile sensors). Until now less attention was paid to the remaining senses. Merely basic investigations were carried out in the field of ’electronic tongues’ [1] whereas gas sensors denoted as ’electronic noses’ [2] are well established under laboratory conditions [3, 4, 5] but have been rarely used in robotics [6, 7, 8] so far. This might be caused by the difficulties that arise when these sensors are used in a real world scenario without the certainty of accurately defined conditions. In con-trast to laboratory experiments, where usually a spe-cial odour delivery system transfers the odour from the source material to the sensor chamber, many pa-rameters like temperature, humidity and strength of flow can vary within a comparably broad range. It is desirable to solve these problems, because there are a lot of possible applications for robots equipped with a sense of ’smell’: First one can imagine an ’electronic watchman’, who is able to detect and lo-calize odours indicating a fire or a chemical haz-ard. Besides handling existing odours, such robots could also use self-produced odourous markers to aid navigation [9, 10] or to communicate with other robots [11].
However, because of the above mentioned prob-lems most publications concerning the use of elec-tronic noses on autonomous mobile robots are con-fined to the task of detecting a known gas rather than recognizing an unknown gas. Furthermore the pub-lished experiments we know of were undertaken in environments with a strong constant airflow (about 0:3m=s) [7, 6] or have been restricted to small
dis-tances between the odour source and the detector [8]. In this way, a major problem that restricts the benefit of the gas sensors which were used in the real world experiments described below, is avoided: Air turbulences, even small indiscernible air move-ments in unventilated apartmove-ments, make it difficult
Figure 1: The autonomous mobile robot ARTHUR equipped with an electronic nose. Notice the tube-shaped sensor sticks, which were mounted at the end of the outstanding bars on the front side of the robot.
to detect spatial and temporal differences in odour molecule concentration [8, 12].
Thus the goal of this work was mainly to demon-strate that a robot equipped with an electronic nose is capable of detecting an odour source in an indoor environment without the presence of a strong con-stant airflow. This should be achieved using rather small odour sources which yield gas concentrations comparable to small puddles of leaking liquid chem-icals. Lacking a constant airflow, it is impossible to localize the odour source by means of the two step strategy supposed by Nakamoto et al. [6] and Rus-sell et al. [7] which involves an upwind-search after plume acquisition. Therefore, a different localiza-tion strategy is proposed in this paper.
2
The Odour Sensing Robot
The commercial odour sensing system VOCmeter Vario2was mounted on ARTHUR, a mobile robot based on the model ATRV-Jr from RWII3(see fig.1). Because of its small size (19126cm
3) the basic
unit could be placed inside the robot’s body. There-fore this device, which is depicted in fig.2 can not be seen in fig.1. It gathers measured values of up to 8 single sensor sticks at a rate of 1 Hz and transfers recorded values to the host computer via an RS232 interface. The sensor sticks, which have been con-nected with the VOCmeter Vario over a thin coax
ca-2www.motech.de/pdf/Vario.pdf (MoTech, Reutlingen,
Ger-many)
3www.rwii.com (RWII, USA)
Figure 2: The commercial odour sensing system VOCmeter Vario2. The basic unit of the used
sys-tem can be seen, which is able to operate up to eight sensor sticks over thin coax cables like those shown in this picture. Notice that this device can not be seen in fig.1 because it is placed inside the robot’s body.
ble (RG158) are tubes 50 mm long with a diameter of 10 mm made of high-grade steel. They not only contain the actual sensor unit but also the required transducing electronics. This system has been cho-sen because of its compact design, low power con-sumption and the facility to easily change particular gas sensors. It provides a flexible setup which, for concerns of the presented investigations, has been optimized to detect a beforehand determined sub-stance.
For experimental simplicity ethanol and acetone have been chosen for the odour source. Experi-ments were undertaken using tin oxide gas sensors (Figaro4TGS2620) because of their high
sensitiv-ity to those analytes. This type of chemical sensor show a decreasing resistivity due to an increasing amount of combustible volatile chemicals in the air. This behaviour is caused by an increasing rate of ox-idation reactions taking place at the heated surface of the chemical sensitive layer. It is important to keep in mind that the functionality of the used metal oxide (MOX) sensors heavily depends on the sur-face temperature and that combustion of some alyte molecules (and therefore consumption of an-alyte material) is necessary to operate this type of sensors.
3
The Experimental Setup
In search of weakly ventilated locations we decided to perform two series of measurements at two dif-ferent locations. First, a vacant apartment in an old building was used. Including a 20m long and 2:5m
wide corridor, not ventilated at all and without any people passing by, this site proved to be well suited for our concerns. Because some vacant apartment odours could be sensed with the human olfactory system we decided to repeat the experiments at a different location to examine possible influences of these uncontrollable odours. The second series of measurements was performed within a similar cor-ridor in a university building. Though the dimen-sions of the corridors are comparable (30m long and 2:2m wide) some minor differences exist between
the two used locations: The corridor at the univer-sity is weakly ventilated, one end is completed by a frequently used door and it was not possible to avoid people walking by (even at late evening hours).
In every run the robot was ordered to drive up and down while keeping track the corridors center. Thus the described scenery framed a one dimen-sional axis on which the odour source was placed as well. Each measuring run started with some ’pa-trol drives’ without an extra odour source to record the odorous background at that time. In fact these reference measurements couldn’t be started until the MOX sensors reached their thermal equilibrium, which requires a warm up time of about an hour. Then a jar containing liquid ethanol or acetone was opened. In order to simulate odour sources with var-ious intensities three different jars were used with an opening area of 20, 60 and 130cm2
First the jars were placed at the end of the cor-ridor. This means that due to the implemented ob-stacle avoidance the minimum distance between the MOX sensors and the odour source equaled approx-imately 50cm.
Using the smallest jar established another exper-iment: The robot is able to roll over this flat con-tainer, thus avoiding the need to stop in front of the odour source. Therefore, further series of experi-ments could realized with an odour source placed in the middle of the corridor and not at the end of it.
In order to optimize the performance of the sys-tem, three sensor positions were tested: The sen-sor sticks could be fixed directly to the robot on the outer end of its rear or front bumper. More-over, the robot was equipped with a pair of ’anten-nae’. These bars made of aluminium were 60cm long. Controlled by common servo motors, they could be rotated about a vertical axis near the robot’s
Figure 3: Chemical concentration measurements taken in a corridor in the university building with a robot moving at a constant speed. The gas sen-sor was placed at the front bumber approximately 30cm in front of the robot’s center of mass. A jar with an opening area of 20cm2filled with acetone
was placed in the center of the corridor (s=0m) at
the time t=0s. The measured values are indicated
by strong dots while the sensors position is drawn in with a thin dotted line.
front end. Up to now, this feature, which enables the halted robot to adjust the sensors, didn’t pro-duce any improvement because it turned out that the received signal gets considerably worse when the robot doesn’t move. Therefore, the ’antennae’ were used as a stiff expansion of the robot or, in other words, the possible sensor positions are ap-proximately 30cm,+30cm and+90cm before the
the robot’s center of mass. Because only 3 MOX sensors were available the experiments were under-taken either using two sensors placed on the anten-nae or all three at one side.
4
Results
Some results are shown in figures 3 - 6. In each diagram the robot position is plotted against the time since the jar containing the liquid chemical was opened. The origin of the distance axis, indicated by a thick dashed line, denounces the position of the odour source. Sensor data, which were normalized to their base level, are plotted into the same graph against a second ordinate. Its scale was calibrated by measurements using a headspace sampler. Though it was not possible to reproduce exactly the same con-ditions in the laboratory as in the real world scenery, the stated values can only serve as a rough represen-tation of concentration differences rather than
abso-Figure 4: Chemical concentration measurements taken in a vacant appartement with a robot moving at a constant speed. The gas sensors were placed at the end of the robot’s ’antennae’ approximately 90cm in front of the robot’s center of mass. A jar with an opening area of 130cm2filled with ethanol was placed at the end of the corridor (s=0m) at the
time t=0s. The measured values are indicated by
strong dots (right sensor) and crosses (left sensor) while the sensors position is drawn in with a thin dotted line.
lute values. Figure 3 shows the result of an exper-iment accomplished in the university corridor. In the lightly shaded left part of this diagram two runs up and down the corridor are shown which were recorded before the jar has been opened, whereas the right part shows several runs in the presence of the odour source placed at s=0m. In all runs the robot
moved with a constant speed (5cm=s - 20cm=s). It
can be seen that the robot clearly senses the pres-ence of the analyte. In this case the smallest jar filled with acetone was used, while similar results were obtained using ethanol or one of the bigger jars. Notice that this result was achieved although the odour source was comparably weak and several people passed by in the course of this experiment.
Results of an experiment undertaken in the vacant appartement are shown in fig.4. The robot moved with a constant speed of 5cm=s and the jar (130cm
2)
was placed at the end of the corridor. This figure shows the minor influence of the environment and that similar results were obtained using ethanol in-stead of acetone. Although the run of the curve is somewhat smoother, the general trend is similar to that shown in fig.3. Approaching the odour source leads to rising sensor values and the resulting peak clearly indicates the position of the maximum con-centration of the analyte. Notice that the slow prop-agation of the volatile analyte is indicated by the
ris-Figure 5: Chemical concentration measurements taken in a corridor inside a vacant appartement with a robot applying a stop-and-go strategy. The gas sensor was placed at the end of one of the robot’s ’antennae’ approximately 90cm in front of the robot’s center of mass. A jar with an opening area of 130cm2filled with ethanol was placed at the end of the corridor (s=0m) at the time t=0s. The
measured values are indicated by strong dots while the sensors position is drawn in with a thin dotted line.
ing baseline either in fig.3 and in fig.4.
The robot moved at a constant speed during the just described investigations. In contrast to that, re-sults of another run in which the robot executed a stop-and-go strategy can be seen in fig.5. The corre-sponding experiments were performed in the above mentioned vacant appartement using the biggest jar (130cm2) filled with ethanol. It is obvious that this strategy is less suitable to solve the detection or even the localization problem. This might be caused by the working principle of metal oxide sensors which involves the consumption of some analyte material. If there is no air movement relative to the sensor the consumed analyte is replaced by molecular diffu-sion only. It is known that the diffudiffu-sion velocities of gases are generally very slow [6]. Thus the satura-tion level of metal oxide gas sensors is significantly reduced if the transport of molecules is governed by diffusion. However, in a real world situation some weak airflow always exists caused by draft or con-vection flow. If the robot stops these rather arbitrary air movements dominate the received signal, super-imposing that signals portion which contains the in-formation on the analytes concentration.
Regarding fig.6 this effect appears even more striking. The diagram shows some subsequent runs which were carried out within the same experiment as shown in fig.5. If the robot moves forth and back
Figure 6: Chemical concentration measurements taken at the same experiment described in fig.5 with a robot moving at a constant speed.
with a constant velocity of 15cm=s the run of the
resulting curve is rather smooth indicating the loca-tion of the odour source by an outstanding peak. The last approach leads to a different behaviour: Imme-diately after the robot stops in front of the jar the sensor signal drops and the curve reveals acyclic os-cillations.
5
Localization Strategy
If the robot drives over an odour source the result-ing curve shows a distinct peak which indicates the point of maximum approximation between the gas sensor and the used jar (fig.3). It is therefore ex-pected that a similar peak will arise if the robot is passing by within a certain distance. This dis-tance is expected to be in the order of several me-ters because the full width at half maximum of the sustained peaks exceeded 5 meters even when the low intense odour source was used. Thus the fol-lowing localization strategy seems to be reasonable: First the robot tries to examine the complete area in search of a peaked sensor signal. Considering the fact that driving with a constant speed is necessary to achieve meaningful sensor values, the examina-tion of the environment should be preferably done by driving on straight lines. It is intended to use an algorithm developed by Kasper et al. [13] to perform the described area-wide search. Once the robot has recorded a discernible peaked curve, it fits the run of the curve by an appropriate function and returns to the center position of the identified peak. After being rotated by 90 degrees the robot examines the whole perpendicular line. If another peaked curve is detected, the middle of this peak is supposed to be
the location of the odour source.
6
Conclusion
The presented results demonstrate that the described odour sensing system is able to detect volatile sub-stances. The investigations showed that even weak odour sources can be sensed and that the influence of the environment is rather negligible.
The experiments revealed that the performance of the mobile odour sensing system could be signifi-cantly enhanced by driving the robot with a constant not too low speed, thus adding an extra airflow rela-tive to the metal oxide sensors.
Furthermore a localization strategy was proposed which takes into account the special behaviour of the used gas sensors. It is intended to test the above drafted localization strategy in a wide rectangular environment like a gymnasium.
Within the scope of our investigations the sen-sors were tested at different positions. Although the results don’t suggest a clearly favorable position a more exposed one seems to be preferable. Addi-tional studies are needed to test other possible meth-ods to improve the quality of the received sensor signal. First trials using different common pc fans yielded no considerable improvements but further experiments are needed to make concluding state-ments.
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